Detecting value of output capacitor in switching regulator

ABSTRACT

An output capacitor of a switching converter filters the triangular current waveform output by an inductor. An auxiliary capacitor, having a capacitance that is much smaller than a capacitance of the output capacitor, is coupled in parallel with the output capacitor so as to conduct a portion of the inductor current. A slope detector circuit determines a slope of the auxiliary capacitor current, and outputs a slope signal corresponding to the slope. A process circuit receives the slope signal and a signal corresponding to the inductor current. Since the auxiliary capacitor current slope is known, along with the auxiliary capacitance value and inductor current, the process circuit can derive the value of the output capacitor by applying a scaling factor. The derived value can be used to dynamically tweak the compensation of the feedback loop or identify a failure of the output capacitor.

FIELD OF THE INVENTION

This invention relates to switch mode power supplies, such as DC/DCconverters, and, in particular, to a technique to determine thecapacitance value of the smoothing output capacitor.

BACKGROUND

FIG. 1 illustrates one type of prior art current mode DC/DC switchingpower supply, also known as a peak current mode DC/DC converter. Theconverter is a buck converter since the output voltage Vout is lowerthan the input voltage Vin. Many other converter types, such as aswitching voltage mode converter, can also benefit from the presentinvention.

The operation of the converter is conventional and is as follows.

A clock (CLK) signal is applied to the set input of an RS flip-flop 20.

The setting of the RS flip-flop 20 generates a high signal at its Qoutput. A logic circuit 24, in response, turns the transistor switch 26on and turns the synchronous rectifier switch 28 off. Both switches maybe MOSFETs or other types of transistors. A diode may replace thesynchronous rectifier switch 28. The logic circuit 24 ensures that thereis no cross-conduction of switches 26 and 28. The input voltage Vinapplied to an inductor L1 through the switch 26 causes a ramping currentto flow through the inductor L1, and this current flows through a lowvalue sense resistor 32. The sense resistor 32 may instead be located onthe other side of the inductor L1. There are various other ways todetect the inductor current. The ramping current is filtered by anoutput capacitor Cout and supplies current to the load 38. The outputcapacitor Cout is relatively large to smooth out ripple.

The output voltage Vout is applied to a voltage divider 42, and thedivided voltage is applied to the inverting input of a transconductanceerror amplifier 44. Capacitors may be connected across the resistors inthe divider 42 to further compensate the feedback loop. A referencevoltage Vref is applied to the non-inverting input of the amplifier 44.The output current of the amplifier 44 corresponds to the differencebetween the actual output voltage Vout and the desired output voltage.The voltage (a control voltage Vc) at a capacitor 46, connected to theoutput of the amplifier 44, is adjusted up or down based on the positiveor negative current output of the amplifier 44. The RC time constant ofthe capacitor 46 and resistor 47 can be adjusted to compensate thefeedback loop to improve stability. The transconductance (gm) of theerror amplifier 44 can also be adjusted to improve stability. Thecontrol voltage Vc, among other things, sets the duty cycle of theswitch 26, and the level of the control voltage Vc is that needed toequalize the inputs into the amplifier 44.

The control voltage Vc is applied to a comparator 50. The rampingvoltage drop across the sense resistor 32, when the switch 26 is on, issensed by a differential amplifier 52, which outputs the voltage Visenseproportional to the inductor current. When the voltage Visense exceedsthe control voltage Vc, the comparator 50 is tripped to output a resetpulse to the RS flip-flop 20. This turns the switch 26 off and turns thesynchronous rectifier switch 28 on to discharge the inductor L1, causinga downward ramping current. In this way, the peak current through theinductor L1 for each cycle is regulated to generate a desired outputvoltage Vout. The current through the sense resistor 32 includes a DCcomponent (the lower frequency, average current) and an AC component(the higher frequency, ripple current).

FIG. 2 shows an example of the ramping voltage Visense (or the inductorcurrent). The DC load current is the average of the triangular waveform.

In some systems powered by the buck converter, it is vital to maintain areliable output voltage. The capacitance of the output capacitor Couttypically reduces with age, stresses, and temperature variations. Thisis especially true when the buck converter is powering high currentequipment, such as servers and motors. When the output capacitorcapacitance reduces, the ripple in the output voltage may exceed adesired amount. Further, when the capacitance reduces, it may result inlarge perturbations in the output voltage during load transients, whichmay not be acceptable for certain loads. Such poor regulation can causeinstability and indicate failure of the output capacitor.

What is needed is a technique for use in a switching converter thatautomatically detects the real time value of the output capacitor. Suchinformation may be used to identify an output capacitor failure or toautomatically adjust the compensation of the feedback loop to improvestability.

SUMMARY

A circuit for deriving the real time value of the output capacitor in aswitching power supply, such as a buck regulator, is disclosed. Theswitching power supply uses an output inductor, which outputs atriangular waveform at the switching frequency. The relatively largeoutput capacitor smooths the waveform to provide a DC voltage to theload.

A small auxiliary capacitor, which may be on the order of 1/1000th thevalue of the output capacitor, is connected in parallel with the outputcapacitor. The slope of the current (positive or negative) into theauxiliary capacitor is determined, and the slope is inverselyproportional to the value of the output capacitor.

The AC current into the output capacitor is approximately the ACinductor current i_(L), so the current into the auxiliary capacitor isapproximately (Caux/Cout)*i_(L). Therefore, the AC current into theauxiliary capacitor is increased as the capacitance of the outputcapacitor goes down. The slope of the auxiliary capacitor current isrelated to the current into the auxiliary capacitor and thus to thecapacitance of the output capacitor. The slope is detected, and thevoltage (Vslope) corresponding to the slope is inversely proportional tothe capacitance of the output capacitor. This Vslope is then used toautomatically optimize the compensation of the feedback loop, ordetermine if the output capacitor has gone below a threshold value, orfor any other purpose.

If the auxiliary capacitor current was subtracted from the inductorcurrent, the slope of the difference signal would be directlyproportional to the capacitance of the output capacitor.

The circuit may be used to greatly increase the reliability of switchingpower supplies.

The auxiliary capacitor current waveform may also be used to identifythe equivalent series resistance (ESR) of the output capacitor. Thederived ESR may then be used to optimize the compensation of thefeedback loop.

Various other embodiments are described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art DC/DC current mode converter.

FIG. 2 illustrates a sample waveform during operation of the converterof FIG. 1.

FIG. 3 illustrates an embodiment of the invention coupled to the outputof a switching power supply, where a voltage directly related to thevalue of the output capacitor is derived from the current into theauxiliary capacitor.

FIG. 4A illustrates the AC current into the large output capacitor inFIG. 3, which is approximately the AC inductor current, at the switchingfrequency.

FIG. 4B illustrates the AC current into the small auxiliary capacitor inFIG. 3, which may be on the order of 1/1000 that of the current into theoutput capacitor. The current into the auxiliary capacitor is shown foroutput capacitor values of 47 μF and 330 μF.

FIGS. 5A and 5B are waveforms of the AC current into the outputcapacitor and auxiliary capacitor, respectively, when the equivalentseries resistance (ESR) of the output capacitor is relatively high. Thecurrent into the auxiliary capacitor is shown for output capacitorvalues of 47 μF and 330 μF. The slopes are the same in FIGS. 4A and 4B.

FIG. 6 illustrates a subtractor that subtracts an auxiliary currentsignal from the inductor current signal, where the difference signal isthen applied to the differentiator in FIG. 3 to generate a slope signalthat is directly proportional to the capacitance of the outputcapacitor.

FIG. 7 illustrates one use of the derived output capacitor value incontrolling the gm of the error amplifier to optimize compensation ofthe feedback loop.

FIG. 8 illustrates another use of the derived output capacitor value indetermining a failure of the output capacitor or that the capacitancehas gone below a threshold level.

FIG. 9 illustrates additional use of the circuit in deriving the ESR ofthe output capacitor from the auxiliary current waveform, where the ESRmay be used to compensate the feedback loop or identify a failure of theoutput capacitor.

Elements that are the same or equivalent are labeled with the samenumeral.

DETAILED DESCRIPTION

FIG. 3 illustrates one embodiment of the invention that can be added toany suitable switching power supply. The detection of the auxiliarycapacitor current is performed while the output capacitor is coupled tothe inductor. Some types of switching power supplies always have theoutput capacitor coupled to the inductor, such as buck converters andforward converters. Other types of power supplies intermittentlydisconnect the output capacitor from the inductor. In such types, thedetection of the auxiliary capacitor current occurs during the timesthat the output capacitor is connected to the inductor. Examples ofpower supplies that may make use of the invention include current modeconverters, voltage mode converters, buck converters, boost converters,buck-boost converters, SEPIC converters, Cuk converters, flybackconverters, forward converters, etc. The inductor L1 and outputcapacitor Cout represent the output inductor and capacitor in anysuitable converter, such as the current mode buck converter of FIG. 1.

The circuit of FIG. 3 basically derives the slope of the current into asmall auxiliary capacitor connected in parallel with the outputcapacitor. The current will typically be a triangular waveform. Theslope is inversely proportional to the capacitance of the outputcapacitor. The slope, corresponding to the derived voltage Vslope, canthen be used to automatically compensate the feedback loop to optimizeit as the output capacitor changes over time, or used to identify if thecapacitance of the output capacitor has degraded below a threshold, orfor any other purpose.

There are many ways to derive the slope of the auxiliary capacitorcurrent, and FIG. 3 just illustrates one way.

In FIG. 3, while the switching regulator is in its steady state, theinductor L1 outputs a triangular waveform, where the average current isthe DC current supplied to the load, such as the load 38 in FIG. 1. TheAC component is filtered by the relatively large output capacitor Cout.The current into the output capacitor Cout is approximately the ACinductor current i_(L). The DC voltage at the output capacitor Cout isthe output voltage Vout of the converter. An example of the AC currentinto the output capacitor Cout is shown in FIG. 4A. The example showsthe current ramp between 1 Amp and −1 Amp.

A much smaller auxiliary capacitor Caux is connected in parallel withthe output capacitor Cout. The value of the auxiliary capacitor Caux canbe very small, such as on the order of 1/100th to less than 1/1000ththat of the output capacitor Cout, so the current into the auxiliarycapacitor Caux (i_(caux)) is approximately (Caux/Cout)*i_(L). Thecurrent into the auxiliary capacitor may be a positive or negativeramping current. An example of the current into the auxiliary capacitorCaux is shown in FIG. 4B. The example shows the auxiliary capacitor Cauxcurrent ramp between 2 mA and −2 mA for an output capacitor Cout valueof 47 μF and a much smaller current ramp between 0.4 mA and −0.4 mA foran output capacitor Cout value of 330 μF. The value of the auxiliarycapacitor Caux is approximately 0.1 μF. As seen by the waveforms, theslope of the auxiliary capacitor current is inversely proportional tothe capacitance of the output capacitor Cout.

The voltage at the bottom terminal of the auxiliary capacitor Caux ismaintained at the AC ground of 0 volts using the op amp 60. Therefore,both the output capacitor Cout and the auxiliary capacitor Caux arecoupled across Vout and 0 volts. The feedback resistor 62 sets the gainof the op amp 60. The the non-inverting input of the op amp 60 iscoupled to ground (or another reference voltage), and the invertinginput is connected to the auxiliary capacitor Caux. The feedback triesto keep the inputs matched so that the bottom terminal of the auxiliarycapacitor Caux is maintained at the AC ground of 0 volts. The output ofthe op amp 60 (Vcaux) directly corresponds to the current into theauxiliary capacitor Caux. The output of the op amp 60 may be atriangular waveform, depending on the ESR of the output capacitor Cout.

The auxiliary capacitor Caux current could also have been detected by acircuit connected between the auxiliary capacitor Caux and the inductorL1, such as by using a low value sense resistor.

The current signal is then buffered by a buffer 64 and applied to adifferentiator 66 to identify the slope of the current waveform. Thedifferentiator 66 comprises a capacitor 68, an op amp 70, and a feedbackresistor 72. The output of the differentiator 66 will be approximately asquare wave. The crest of the square wave is a voltage corresponding tothe downslope of the current ramp output by the op amp 60, and thetrough of the square wave is a voltage corresponding to the upslope ofthe current ramp. The slope magnitudes may be the same but of oppositepolarity.

A sample and hold circuit 74 samples the slope values at the clock rateof the converter, such as using a delayed CLK signal from the clock inFIG. 1. In the example of FIG. 3, the sampling switch 76 is closed for abrief moment by the delayed CLK signal, delayed by the delay circuit 78.This sampled value is stored in a hold capacitor 80 until the nextsampling time. The sample and hold circuit 74 may also include adischarge circuit for resetting the capacitor 80 voltage prior to eachsampling time.

The output of the sample and hold circuit 74 is a voltage Vslope that isproportional to 1/Cout. The actual value of the output capacitor Coutcan therefore be simply calculated by applying a scaling factor toVslope. The scaling factor takes into account the inductor current, thevalue of the auxiliary capacitor Caux, the relationship between slopeand the auxiliary capacitor Caux current, and the various gains in thesystem. The proper scaling factor may be determined by simulation.

A process circuit 84 receives a signal corresponding to the inductorcurrent, such as the Visense signal in FIG. 1. The Vcaux signal (outputfrom the op amp 60) and Visense must be properly scaled to reflect theactual ratio of the inductor current to the auxiliary capacitor current.If scaling is required, it may be done using a voltage divider or byother methods. The process circuit 84 is designed for a particular valueof the auxiliary capacitor Caux. Since Vslope has a known relationshipto the current into the auxiliary capacitor Caux, and the inductorcurrent and the value of the auxiliary capacitor Caux are known, thenthe following equation can be solved for Cout: Cout=Caux*i_(L)/i_(caux).

The process circuit 84 can use digital or analog hardware or a processorto derive a voltage corresponding to the value of the output capacitorCout. A lookup table may also be used that receives the inductor currentsignal and Vslope and outputs the output capacitor Cout value or someother signal corresponding to the real time value of the outputcapacitor Cout. The process circuit 84 basically multiplies Vslope by ascaling factor to generate an output signal corresponding to the realtime value of the output capacitor Cout. Controllable scaling circuitsare well known. One skilled in the art can easily determine which typeof process circuit 84 is best to use for a particular application.

FIGS. 5A and 5B are waveforms of the current into the output capacitorCout and auxiliary capacitor Caux, respectively, when the equivalentseries resistance (ESR) of the output capacitor is relatively high. Theslopes are the same as in FIGS. 4A and 4B but there is a DC voltage dropcause by the current through the equivalent series resistor (ESR) of theoutput capacitor Cout which shifts the waveform up or down. Only theslopes are relevant, which do not change with ESR.

In another embodiment, the auxiliary capacitor Caux current issubtracted from the inductor current, and the slope of that differenceis derived. The slope is then proportional to the output capacitor Coutvalue and is thus used to determine the real time value of the outputcapacitor Cout. FIG. 6 shows a circuit that may be inserted before thedifferentiator 66 in FIG. 3 for generating the difference signal. Theauxiliary capacitor Caux current is processed by the op amp 60 andbuffer 64 of FIG. 3, and the corresponding voltage signal Vcaux isapplied to a subtractor 86, which subtracts Vcaux from the inductorcurrent signal Visense (FIG. 1). The two signals must be scaled properlyto accurately reflect the ratio of the inductor current to the auxiliarycapacitor current. The resulting difference signal is then processed bythe differentiator 66 and sample and hold circuit 74 to generated aVslope signal that is directly proportional to the capacitance of theoutput capacitor.

The output of the process circuit 84 may be used for any purpose. Onepurpose is shown in FIG. 7, where the output of the process circuit 84,corresponding to the real time value of the output capacitor Cout, isused to control the transconductance (gm) of the error amplifier 44 ofFIG. 1 to optimize the gm for stabilizing the feedback loop. The outputcould have also been used to tweak any compensation RC circuit in thefeedback loop. This allows the user to use a wide range of outputcapacitor Cout values without adversely affecting the stability of theconverter. For example, the user may want to use a relatively smalloutput capacitor Cout due to size or cost limitations, and the processcircuit 84 would automatically adjust the compensation circuit for thatparticular output capacitor Cout.

FIG. 8 illustrates an example where the process circuit 84 or anothercircuit determines that the capacitance of the output capacitor Cout hasfallen below an acceptable threshold. As a result, a warning signal maybe issued and the output capacitor Cout is replaced. This is a verysignificant problem for high power converters that power vitalequipment, such as servers, where there is a lot of stress on the outputcapacitor that accelerates its aging. Aging will typically cause thecapacitance value to be reduced over time, which increases the ripple ofthe converter's output voltage. Extremely low temperatures may alsocause the output capacitor to have an unacceptably low capacitance. Todetermine whether the capacitance has fallen below a threshold, theprocess circuit 84 derives a signal corresponding to the outputcapacitance value and compares it to a threshold value using acomparator.

Although the example illustrates a differentiator 66 to determine theslope, other circuits may be used to calculate the slope, such as acircuit that subtracts two samples of the ramp signal and divides thedifference by time.

As shown in FIG. 5B, the DC shift of the auxiliary capacitor current, ifany, can be detected to determine the ESR of the output capacitor Cout.The ESR is proportional to the shift. As shown in FIG. 9, an additionalprocess circuit 90 is used to detect Vcaux (a voltage corresponding tothe auxiliary capacitor current) and then derive the ESR of the outputcapacitor Cout. The ESR adds perturbations to the output voltage Vout ofthe converter. The derived ESR may be used to improve compensation ofthe feedback loop or identify a failure of the output capacitor Cout.

Since inductors and output capacitors are typically relatively largecomponents, they are often provided external to the integrated circuitthat contains the control circuitry for the switching power supply.Multiple output capacitors may be connected in parallel to provide thedesired capacitance, and the combination of such capacitors isconsidered to be a single output capacitor. Since the auxiliarycapacitor Caux and its related circuitry in FIG. 3 can be very small, itmay be located on the same die as the control circuitry for theswitching power supply and so implementing the invention in a switchingpower supply incurs no additional cost. For example, the combinedcircuits of FIGS. 1 and 3, except for the RC compensation network, load,voltage divider, inductor L1 and output capacitor Cout, may be providedon a single chip.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications thatare within the true spirit and scope of this invention.

What is claimed is:
 1. A circuit for use with a switching converterhaving an output capacitor for filtering a current output by aninductor, the circuit comprising: an auxiliary capacitor having acapacitance that is smaller than a capacitance of the output capacitor,the auxiliary capacitor configured to be coupled to a first terminal ofthe output capacitor so as to conduct a portion of the current output bythe inductor, the portion being an auxiliary capacitor current having aslope; a slope detector circuit coupled to receive a first signalcorresponding to the auxiliary capacitor current to determine a slope ofthe auxiliary capacitor current, the slope detector circuit foroutputting a slope signal corresponding to the slope; and a processcircuit configured to receive the slope signal to derive a second signalcorresponding to a value of the output capacitor.
 2. The circuit ofclaim 1 wherein the slope detector circuit comprises: a differentiatorfor receiving the first signal corresponding to the auxiliary capacitorcurrent and outputting a slope waveform corresponding to the slope ofthe auxiliary capacitor current; and a sample and hold circuit forsampling the slope waveform for generating the slope signal forprocessing by the process circuit.
 3. The circuit of claim 1 wherein thecapacitance of the auxiliary capacitor is less than one-hundredth thecapacitance of the output capacitor.
 4. The circuit of claim 1 whereinthe process circuit is configured to receive, along with the slopesignal, an inductor current signal proportional to the current output bythe inductor.
 5. The circuit of claim 4 wherein the process circuit isconfigured to use the known capacitance of the auxiliary capacitor, theinductor current signal, and the slope signal to derive the secondsignal corresponding to the value of the output capacitor.
 6. Thecircuit of claim 1 wherein the circuit is formed on a single die alongwith control circuitry for the switching converter.
 7. The circuit ofclaim 1 wherein the auxiliary capacitor is coupled between the firstterminal of the output capacitor and a reference voltage.
 8. The circuitof claim 7 wherein the reference voltage is an AC ground.
 9. The circuitof claim 7 wherein the auxiliary capacitor has a first terminalconfigured to be coupled to the first terminal of the output capacitorand a second terminal coupled to an inverting input of an op amp,wherein a non-inverting input of the op amp is coupled to the referencevoltage, and wherein an output of the op amp is fed back to theinverting input as a feedback signal, whereby the feedback signalmaintains the second terminal of the auxiliary capacitor atapproximately the reference voltage.
 10. The circuit of claim 1 whereinan output of the process circuit controls parameters of a feedback loopin the switching converter.
 11. The circuit of claim 10 wherein theoutput of the process circuit controls a transconductance of an erroramplifier in the switching converter.
 12. The circuit of claim 10wherein the output of the process circuit tweaks the feedback loop toimprove a stability of the switching converter.
 13. The circuit of claim10 wherein the output of the process circuit tweaks a compensationcircuit in the feedback loop to improve the stability of the switchingconverter.
 14. The circuit of claim 1 wherein an output of the processcircuit identifies whether the capacitance of the output capacitor hasfallen below a threshold.
 15. The circuit of claim 1 wherein the slopesignal is a voltage proportional to 1/Cout, where Cout is thecapacitance of the output capacitor.
 16. The circuit of claim 1 whereinthe slope signal is a voltage proportional to Cout, where Cout is thecapacitance of the output capacitor.
 17. The circuit of claim 1 whereinthe process circuit multiplies the slope signal by a scaling factor toderive the second signal corresponding to the value of the outputcapacitor.
 18. The circuit of claim 1 wherein the process circuit is afirst process circuit, the circuit further comprising: a second processcircuit coupled to receive the first signal corresponding to theauxiliary capacitor current and derive an equivalent series resistance(ESR) of the output capacitor.
 19. The circuit of claim 18 wherein thesecond process circuit scales a DC voltage shift in the first signal,corresponding to the ESR, to derive the ESR.
 20. The circuit of claim 1further comprising the switching converter, the switching convertercomprising: one or more switching transistors for supplying current tothe inductor; control circuitry for controlling a duty cycle of the oneor more switching transistors to output a target output voltage to aload; and the output capacitor for smoothing the current output from theinductor.
 21. The circuit of claim 1 wherein the switching converter isa buck converter.
 22. A method performed by a switching convertercomprising: controlling one or more switching transistors for supplyingcurrent to an inductor, the inductor outputting a ramping currentwaveform; controlling a duty cycle of the one or more switchingtransistors to output a target output voltage to a load; smoothing theramping current waveform using an output capacitor; detecting a slope ofan auxiliary capacitor current, wherein an auxiliary capacitor having acapacitance that is smaller than a capacitance of the output capacitoris coupled to a first terminal of the output capacitor so as to conducta portion of the current output by the inductor, the portion being theauxiliary capacitor current having the slope; detecting the slope of theauxiliary capacitor current, and outputting a slope signal correspondingto the slope; and processing the slope signal to derive a capacitancevalue signal corresponding to a value of the output capacitor.
 23. Themethod of claim 22 wherein detecting the slope comprises: using adifferentiator for receiving the auxiliary capacitor current andoutputting a slope waveform corresponding to the slope of the auxiliarycapacitor current; and sampling the slope waveform for generating theslope signal.